1. Field of the Invention
This invention relates to a stroke indicator in an air-operated combination diaphragm spring brake actuator of the type used with air brake systems on vehicles such as trucks. More specifically, the invention relates to a stroke indicator which signals a maximum limit of stroke attained by the actuator push rod.
2. State of the Prior Art
Spring-applying brake actuators are in common use with air brake systems used on trucks, buses, and towed vehicles. For normal braking operations, such actuators are typically provided with a service chamber, which apply and release the brakes in response to the operator directed delivery and exhaust of compressed air. For parking or emergency brake operation, such actuators are typically provided with a spring chamber disposed in tandem with the service chamber, which actuate the service chamber by exhausting air from the spring chamber.
In a typical air brake, the spring chamber and the service chamber are separated by a divider wall, in which is disposed a sealed annular opening. The seal typically comprises one or more O-rings positioned with grooves machined in the annular opening. The service brake chamber has a diaphragm that abuts a service push rod and separates the service push rod from the divider wall. Pressurized air is delivered or exhausted into the service chamber between the divider wall and the diaphragm to move the service push rod and apply or release the brakes.
The spring chamber has a diaphragm that abuts a spring plate. The spring plate separates the diaphragm from a power spring. An adapter push rod abuts the diaphragm and passes through the sealed opening of the divider wall. The O-rings of the sealed opening contact the exterior of the adapter push rod to prevent the transfer of air between the service chamber and the spring chamber.
Generally, a barrel-shaped power spring is used and provides the stored energy necessary to exert the large force required for emergency braking in the event of air pressure failure or as a parking brake. When the parking brake or emergency brake is not in use, pressurized air acting on the diaphragm compresses the spring to maintain it in its brake release position. When the air is exhausted from the spring chamber, the spring acts on the spring plate and diaphragm, which forces the adapter push rod into contact with the diaphragm of the service chamber and the service chamber push rod to apply the parking or emergency brakes.
As the vehicle's brakes are applied over time, the brake pads are worn away, resulting in an increase in the stroke of the service push rod needed to apply the necessary braking force. The stroke of the service push rod can also change if the brakes are out of adjustment. Thus, the length of travel of the service push rod is an indicator of brake wear or improper brake adjustment. The stroke of the service chamber push rod has long been used to indicate brake wear or improper adjustment because the service chamber push rod is easily visible, whereas the brake pads and brake assembly cannot easily be inspected because of the complexity and location of the brake pads and brake assemblies.
The prior art discloses many attempts to use the stroke of the service push rod to indicate brake wear and improper adjustment. Prior art stroke indicators include an electric switch disposed within or outside the brake housing and associated with the service push rod or service push rod plate to activate a signal on the vehicle dashboard when the brakes are worn past a predetermined level or out of adjustment. Examples of such electronic indicators are the U.S. Pat. No. 2,914,630 issued Nov. 24, 1959; U.S. Pat. No. 4,207,565, issued Jun. 10, 1980, U.S. Pat. No. 4,583,071, issued Apr. 15, 1986; U.S. Pat. No. 4,757,300, issued Jul. 12, 1988; and U.S. Pat. No. 4,800,991 issued Jan. 31, 1989.
The prior art also includes mechanical stroke indicators. U.S. Pat. No. 4,279,214, issued Jul. 21, 1981 and U.S. Pat. No. 4,989,537 issued Feb. 5, 1991 disclose a mechanical stroke indicator comprising a sleeve which surrounds the push rod and which is urged outwardly from the brake housing by the push rod plate when the brake is applied. The sleeve can have indicia that indicate the amount of stroke. A metal extension associated with the push rod plate and passing through the service brake housing is used as a stroke indicator in U.S. Pat. No. 3,298,466, issued Jan. 17, 1967. The amount of the plate that extends beyond the brake housing indicates the amount of stroke. U.S. Pat. No. 4,991,310, issued Feb. 12, 1991; U.S. Pat. No. 5,044,302, issued Sep. 3, 1991; and U.S. Pat. No. 5,140,932, issued Aug. 25, 1992 disclose rulers having indicia and extending from the brake housing, parallel to the push rod are used in combination with a pointer mounted on the push rod to indicate the stroke.
Indicia associated with the service brake push rod have also been used as stroke indicators. The U.S. Pat. No. 4,945,818, issued Aug. 7, 1990, disclosed indicia etched or cut into the surface of the service brake push rod. If the indicia were visible after setting the parking brake, the brake pads were sufficiently worn or improperly adjusted to warrant inspecting the brakes. Adhesive labels having indicia have also been applied to the push rod to indicate the amount of stroke.